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Thread: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

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    Default How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Not everyone likes parachute sea anchors. One unnamed, famous monohull sailor seriously criticized the use of parachute sea anchors because he said that if a breaking wave hits the bow of a yacht and pushes the yacht backwards, the strains on the rudder may rip the rudder off the yacht or seriously damage the rudder. Because he is a yacht designer and an experienced offshore sailor, it's probably worth examining his point of view.

    We use 500 feet of Samson 1 inch double braided nylon for our parachute tether on Exit Only. http://www.cbknot.com/2in1GoldNBraid.htm

    I have been told that this double braid has up to about ten percent stretch, which means that 500 feet of double braid could stretch as much as fifty feet. A wave strike on the bow could possibly push my boat backward as much as fifty feet.

    That's the stretch part of the theory, and there is probably some merit to it. However, when you are lying to a parachute, you are already taking some stretch on the double braid (there's catenary to think about as well). The truth is, I don't know how far backwards my boat could be shoved by a wave strike on the bow, but it could be substantial.

    Multihulls lie to a parachute different (better) than a monohull, and a bridle usually holds the bows of a catamaran directly into most of the oncoming seas. Since the bows will pierce the oncoming seas rather than experience and transmit their full energy to the rest of the yacht, multihulls have an advantage when lying to a parachute. The Parachute bridle places the cat in the most favorable attitude toward oncoming seas. A sea that might give a monohull a massive shove aft, could have substantially less effect on a cat.

    Rudders come in many different designs. For example, later generation Prout rudders are hung behind a skeg with the rudder post on the leading edge of the rudder. If you push a rudder of this design backwards when it is not fully centered, the rudder may instantly slam to one side creating damage. It's like a barn door caught in a wind.

    Rudders that hang on the stern similarly must be fixed amidships so that they don't get slammed to one side if the catamaran suddenly moves backwards after a wave strike on the bows.

    Balanced rudders are in a much better position if your boat get's pushed backwards by a wave. Since the rudder is balanced, the loading on the rudder may be less since water is flowing across the rudder both in front of and behind the rudder post. Even with a balanced rudder, it's important to lock the wheel steering to decrease the stress on the rudder when lying to a parahcute.

    We used a parachute only one time on Exit Only in a storm north of New Zealand, and we rode out the storm without a problem. We simply locked both steering wheels with the rudders amidship, and there were zero problems with the rudders.

    When you used a parachute sea anchor on your yacht, did you have any problems with your rudders or damage to your rudders? What type of rudders do you have? Do you know of any multihull sailors who have experienced rudder damage while lying to a parachute sea anchor? What type of rudders did they have?
    Last edited by Maxingout; 2nd November 2008 at 01:49 AM.

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    The Pardey's in a monohull lay at an angle to the parachute and leave a slick behind them which they claim calms the sea and stops breaking waves - I believe what they say on that.

    When you lay to the parachute were you dead straight onto it?

    Was there any slick left? Did any breaking waves hit you?
    Safe Sailing
    Paul
    Blog: www.suliere.com

  3. #3

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    The sea anchor bridle held our bows straight into the oncoming seas. We had no breaking seas come on board. Our decks remained dry. We simply bobbed like a cork on the surface of the sea.

    The Pardeys also comment that the parachute sea anchor itself will create turbulence in the water that decreases the power of oncoming seas. It's as if there is an alley of calmer water between you and your parachute to windward.

    Their surmise is plausible, but I have never been in large breaking seas, so I cannot comment.

    I've seen the calm wake behind freighters in extremely rough seas, and to the extent that the parachute simulates the freighter's wake, it might provide protection to a multihull hiding 500 feet downwind directly behind a parachute.

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    One day I was trying to explain the Pardy's smooth wake stopping breaking seas to Lesley. She looked out of the window (we live on the side of a lake) and said "you mean like that small rowing boat there" and pointed to a 12 foot dinghy drifting with two anglers fly fishing. Behind them was a calm slick of water about 60 feet long!!

    So it seems that any object that causes displaces some water and drifts leaves behind it a calmer patch and I can really appreciate what the Pardy's say.

    I now see that slick all the time everywhere - I must have been blind before.

    On your storm chat on the video you mention choosing the right quarter of the storm to sail as part of the storm management but you do not expand on it. Can you please expand a little on that?
    Safe Sailing
    Paul
    Blog: www.suliere.com

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    Quote Originally Posted by ForumAdmin View Post
    On your storm chat on the video you mention choosing the right quarter of the storm to sail as part of the storm management but you do not expand on it. Can you please expand a little on that?
    Weathering a storm at sea is more about the waves than it is about the wind. Your catamaran can withstand wind of almost any strength if you are under bare poles. The same is not true of the seas. Big seas, breaking seas, seas coming from all directions, and seas that constructively interfere with each other to create monster waves can easily initiate a demolition derby on board your yacht.

    The primary focus of storm management needs to be on the seas rather than the winds.

    When a rotating storm heads your way, you need to do everything possible to place your yacht in a location where the seas will be the most manageable.

    Bernard Moitessier used an interesting storm management technique when he sailed Joshua around the world in the high southern latitudes. When an overtaking low approached his boat, he would sail north or north east until the low passed under him, and then he turned south east to get back on course. Using this strategy, he noticed that he experienced less mountainous seas, and overall the seas were less confused and more manageable. Many people believe that constructive interference by converging wave trains is a major contributing factor to mountainous seas. If you can go where the waves arenít, you push the odds of survival in your favor.

    Classically, the teaching is that there is dangerous semicircle and a safe or navigable semicircle in a major rotating storm. On the dangerous side, there is an additive effect of the stormís wind with the forward motion of the storm creating stronger winds. On the navigable side, there is a subtractive effect of the storm's forward motion being deducted from wind speed to create weaker winds on that side of the storm. That is all well and good, but it misses the point to a certain degree. Less wind is better, but since itís the seas that create the demolition derby, the focus should really be on the seas.

    The absolutely worst thing that you can do in a storm is run directly downwind. If you do that, you will sail into the center of the storm where the waves are absolutely chaotic. Waves will be coming at you from every direction because the winds are pushing waves into the center of the storm from every direction. When those waves destructively interfere with each other, your yacht will disappear into giant troughs, and when the waves constructively interfere with each other, your yacht will experience giant seas.

    The first goal in storm management is to get as far from the center as you can, by whatever means you can, for as long as you can, so that you can distance yourself from those chaotic seas. You must never run directly downwind in a rotating storm.

    Fortunately, the power of a storm drops off rapidly the farther away you are from the center. The constructive and destructive interference from converging wave trains will also decrease the farther you are from the storm's center.

    When a storm heads your way, a novice might be tempted to sail in front of a storm to get to the safe semicircle where there is less wind. This is an invitation to disaster. I observed chatter on a cruising forum where they were debating the best course of action because a storm was heading their way, and some people who were north of the stormís path advocated sailing to the south which required sailing across the stormís path. Probably not a good idea.

    For me, the safe quadrant is the quadrant that is farthest from the storm's center. When you look at your position and the stormís path, it should be instantly clear what you need to do. Using weather fax and all the weather resources that are available to us, we have a fairly good idea of the weather heading our way. Forecasters watch the paths of tropical storms and hurricanes like a hawk. Once you know where the storm is headed, you know your safe quadrant, and you know what to do. You sail at right angles to the path of the storm for as long as you can, and after that you put your parachute in the water and hang on. What you donít do is get out your drogue and run downwind toward the stormís center.

    Hereís my strategy on Exit Only.

    1. Waves destroy yachts. Wind doesnít destroy yachts.
    2. Waves are worse closer to the stormís center.
    3. Waves are progressively less severe the farther you are from the stormís center.
    4. Never sail in front of a storm to get to the other ďsafe sideĒ.
    5. Never run downwind in a rotating storm.
    6. Sail perpendicularly away from the stormís path.
    7. Prepare/rig your parachute sea anchor and bridle ahead of time when trouble is brewing so that you can quickly and easily deploy it before conditions get out of control.
    8. Sail as long as you can, as fast as you can, as far as you can from the storms center. When you can sail no farther because conditions are too rough, deploy your parachute.
    9. I use drogues only when running downwind in a non-rotating storm of short duration. I realize that if I run downwind in a rotating storm, I am attempting nautical suicide by running toward the center of the storm.
    10. Stick to the plan.

    Undisciplined thinking without a clearly defined plan can lead to disaster in dealing with storms at sea. Wishful thinking rarely makes your life better, and in a storm at sea, it does no good at all. When you are scared and your mind is numb from exhaustion, itís time to stick to your written plan. Sticking to a well-defined plan increases the odds of surviving storms at sea.

    This is the approach that I use on board Exit Only. I donít claim that everyone should do the things the way that I do them, or think about them in the same way that I do. Itís not the gospel truth. Itís simply what works for me.

  6. #6

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    I can easily and quickly raise my rudders out of the water. I'd probably do this if I was lying to a sea anchor and concerned about them.

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    A balanced rudder of the same area as an unbalanced rudder will give lower loads, that is clear.

    The key factor in designing the rudders is also the strength. John Shuttleworth reccomends that the rudder be designed for 25 knots boat speed with the rudder at right angles!

    This is probably a much higher load than most other designers use, but sound advice IMO.

    I have noticed that Chris White always uses Titanium rudder shafts, also very strong and light.

    Alan

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by ForumAdmin View Post
    On your storm chat on the video you mention choosing the right quarter of the storm to sail as part of the storm management but you do not expand on it. Can you please expand a little on that?
    You need to understand two major things about the cyclones - Buys Ballots law, and that these systems revolve in different directions in the northern hemishere to the southern.

    In the Northern Hemisphere, stand with your back to the wind; the low pressure area will be on your left. This is because wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere, but the angle between the pressure gradient force and wind is not a right angle in low latitudes.


    These documents will help:
    Attached Files Attached Files
    Insanity is continuing to do the same thing and expecting different results

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Yes there is an excellent write up on it in the last chapter of Storm Tactics.

    Basically in the northern hemisphere the safe side of a tropical storm is the southern side nearer the equator

    In the southern hemisphere its the northern side again the one nearer to the equator.

    I am just updating my weather reading to the tropics
    Safe Sailing
    Paul
    Blog: www.suliere.com

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by Nordic View Post
    The key factor in designing the rudders is also the strength. John Shuttleworth reccomends that the rudder be designed for 25 knots boat speed with the rudder at right angles!
    An interesting aside, I recently read that a rudder will actually experience more lateral force just before stall than at right angles to flow due to the large amount of lift generated.

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by Maxingout View Post
    Balanced rudders are in a much better position if your boat get's pushed backwards by a wave. Since the rudder is balanced, the loading on the rudder may be less since water is flowing across the rudder both in front of and behind the rudder post.
    Not exactly. I've learned a lot about rudders in my continuing project of having my rudder blades replaced.

    For the common NACA 0010 foils used on modern cruising boats, a "balanced" rudder has its post (turning axis) positioned with only approx. 17% of the blade area forward of the post. 20% max. This of course means approx. 80 to 83% of the surface area is behind the post meaning that if the normal flow direction is reversed - when the rudder is pushed backwards - there's very little benefit of decreased loads due to opposing loads forward of the post.

    2 Hulls Dave

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by 2hulls View Post
    ....snip .... 20% max. This of course means approx. 80 to 83% of the surface area is behind the post meaning that if the normal flow direction is reversed - when the rudder is pushed backwards - there's very little benefit of decreased loads due to opposing loads forward of the post.2 Hulls Dave
    I find this difficult to agree with.

    force acting on the rudder
    - behind the blade is reduced by 20% due to 20% reduction in blade area.
    - plus 20% counterforce due to the area in front of the blade

    doesnt that mean that there is a 40% reduction in turning force on the blade , thus reducing the effort to hold the blade straight.

    There will of course be no reduction in bending force on the rudder post on a rudder that is hard over.
    Insanity is continuing to do the same thing and expecting different results

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    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by Talbot View Post
    I find this difficult to agree with.

    force acting on the rudder
    - behind the blade is reduced by 20% due to 20% reduction in blade area.
    - plus 20% counterforce due to the area in front of the blade

    doesnt that mean that there is a 40% reduction in turning force on the blade , thus reducing the effort to hold the blade straight.

    There will of course be no reduction in bending force on the rudder post on a rudder that is hard over.
    There is SOME reduction of turning force afforded by balanced rudders going backwards. I did over-simplify the reason, however, by only mentioning surface area - as that is the parameter used to locate an axis to obtain balance. There's more do it than that in the physics.

    Remember that in normal, forward flow situations of the foil designs we typically have, a balanced axis is located at 17 to 20% of the surface area behind the leading edge. So right there, this doesn't make sense considering surface area only. If surface area were the only consideration, the balance would be at 50%.

    The REAL parameter is distribution of pressure on the surface area, not surface area alone. Max water pressure intuitively occurs at max rudder angle. (Normal max angles are about 35% because at greater angles, typical rudders stall.) Because water is striking the rudder blade from ahead, the leading edge does more work than the trailing edge. This is true of all hydrofoils and air foils as well. Thus, the center of water pressure doesn't fall at the horizontal centroid of the rudder surface area, but at some point forward of this. Although the position of the center of pressure moves around with changes of rudder angle, it usually falls somewhere between 30 to 40% aft of the leading edge at normal max rudder angle. Using this as the centroid of pressure, and dividing it up equally fore and aft for location of a balance axis, results in a location of 15 to 20% aft of the leading edge. Because it's not a linear pressure distribution, an arithmetic half is not exact, and 17 to 20% of surface area aft, in practice, has provided the best results for a balanced axis dimension. Your mileage may vary.

    2 Hulls Dave

  14. #14

    Default Re: How Important Is Rudder Design When Lying to a Parachute Sea Anchor?

    Quote Originally Posted by ForumAdmin View Post
    The Pardey's in a monohull lay at an angle to the parachute and leave a slick behind them which they claim calms the sea and stops breaking waves - I believe what they say on that.
    From the FWIW department:

    Must jump in here with an important clarification/correction. Not being pedantic about it. The Pardeys point out and emphasize that it is the slick formed to the WINDWARD side of the BOAT that is important. It can be shown, and I have seen that FLOW DISTURBANCES MOVE UPSTREAM. I know this is counter intuitive, but it is observable fact, and IIRC from engineering school of many years ago, it is provable mathematically.

    Everyone expects the calm water on the downstream side of any impediment to fluid flow. This is readily observed as it can be larger than the calm area to windward, depending upon the shape of the impediment to the flow. But look upstream and you will see a more calm area of water too. This phenomina is observal in other systems that can be modeled as fluid flow, for example automobile traffic on the large motorways. A vehicle stops in midstream and the vehicles make adjustments to their direction of travel increasingly farther upstream as each driver becomes aware that those ahead of him are doing so even though he cannot see the impediment.

    However, the dangerous water is coming at you from the WINDWARD SIDE, and it is this delightful benefit of physics, the WINDWARD SLICK that saves your bacon and your boat. As the Paraanchor is likely one to two waves ahead of the boat and is submerged, it is not the direct cause of the slick to windward of the boat. Indirectly, it is responsible as it causes the boat to be an impediment to the streamlines of the fluid flow.

    Thus, the reason that the Pardeys recommend arranging the boat at a 45 to 50 degree angle to the waves. This provides a greater impediment to the flow than being bow on, without providing an opportunity for the boat to be rolled due to height of the seas.

    In my old steel monohul, hove to the in the north atlantic, I have noticed the area of calmer water to windward as the vessel lay comfortably at a roughly 45 degree angle to the seas. Sometimes, when I backed sail and helm with the boat pointing a bit too high, she would shiver and shake and shudder a bit until she had fallen off and settled at the comfortable wider angle

    Some years ago, on another forum or in some publication (do not recal where or when), there was a good discussion regardihng whether monos and multis should adopt the same strategy. The conclusion was that multis could possibly be successful with the same tactic when resorting to a parachute anchor, but that it might not be as comfortable due to the fact of multiple hulls and rocking effect of passing seas. Other discussions ensued about the loads on the cats as they have more surfaces presented to seas and the conclusion as best I recall was to follow the direct on approach with bridle as outlined by Dave in his approach with Exit Only.

    My $.02 worth.

    Cheers,
    Jim

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